"It's pointless to try to convince him of anything. He's gone off the rails. We're in Crazytown. I don't even know why any of us are here. This is the worst job I've ever had," Kelly was quoted as saying in Woodward's book on the Trump White House.

Kelly denied he had made the comments at the time, but rumours persisted that the relationship between the two was suffering.

Kelly's exit marks the latest in a long list of advisers and cabinet members to come and go from the White House under Trump.

It is the second top-level personnel change Trump has made since the mid-term elections on November 6, after Attorney General Jeff Sessions resigned last month at Trump's request.